This invention relates to the area of oil and natural gas exploration and, more particularly, to a method for identifying regions of sand or sandstone formations having significant water saturations from which hydrocarbons may be produced without significant attendant water production.
Subsurface reservoirs of natural gas and petroleum, hereinafter referred to generically as "hydrocarbons" are typically found trapped in permeable geological strata beneath a layer of impermeable strata material. A hydrocarbon will "float" upon any ground water present although typically, a transition zone will exist between the two fluids due to the water being raised by capillary action of the permeable strata material. In some regions, impermeable layers may be relatively closely stacked atop one another trapping thin zones of what may be essentially hydrocarbons, essentially water or mixed hydrocarbons and water. A well bore dropped through the formation and various layers may produce water if tapped in a transition region or mixed hydrocarbon and water zone. The cost of transporting, separating and disposing of the attendant water adds sufficiently to production costs that hydrocarbon reservoirs have often been left untapped where it is expected or believed they would produce an excessive amount of attendant water.
Water saturation present at various levels of a formation is typically determined from interpretation of conventional electrical (i.e., resistivity) logs taken through a borehole dropped through the formation. Water saturation of the available pore space of the formation is determined from the resistivity log measurements using the Archie equation: EQU S.sub.w.sup.n =aR.sub.w /.phi..sup.m R.sub.t ( 1)
Where "S.sub.w " is the fractional water saturation (i.e. free and bound water of the formation expressed as a percent of the available pore space of the formation), "a" is a formation resistivity coefficient, "R.sub.w " is the formation water resistivity, ".phi." is the formation porosity, "R.sub.t " is the formation resistivity indicated by the resistivity log, "n" is the saturation exponent and "m" is the porosity or cementation exponent. The Archie equation may be expressed in other ways and there are numerous methods in the art for determining, measuring or otherwise obtaining the various components needed to predict fractional water saturation S.sub.w from the log-indicated resistivity, R.sub.t, using the equation in any of its forms.
Low resistivity pay sands are subsurface sand or sandstone formations which indicate low formation resistivity when logged with conventional equipment, implying the presence of a significant amount of water, but produce water-free or relatively low water cur oil or natural gas (i.e. "pay") when tapped. Such sands are located in the Gulf of Mexico and other areas. See for example, E. A. Vajnar, et al "SURPRISING PRODUCTIVITY FROM LOW-RESISTIVITY SANDS," 18th Annual Logging Symposium Transactions, Society of Professional Well Logging Analysts, June 5-8, 1977, Houston Tex.; and J. C. Gauntt et al "The Use of Core Analysis Data to Explain the Abnormally Low Resistivities of Some Hydrocarbon-Productive Simpson Series Sand in Central Oklahoma", 4th Annual Logging Symposium Transactions, Society of Professional Well Logging Analysists, May 23-24, 1963, Oklahoma City, Okla. discussing this phenomena.
Hereinafter "sand" will be used generically to refer to both consolidated and unconsolidated subsurface geological formations of that material. Also "log" will be used to refer to a record of one or more of the characteristics of a formation which varies with depth, including the signals generated by an appropriate logging tool while traversing the formation, as well as to values derived from such signals or several sets of signals. Thus, for example, a porosity log may be derived for a formation from a density log obtained by logging the formation with a density logging tool. The record may have a temporary form like a CRT display or values in a computer memory or a permanent form like a magnetic tape or strip chart.
Several methods ave been proposed to identify low resistivity pay sands. Most involve determining the bound or irreducible water saturation of the formation from laboratory measurements of samples of the formation material and comparing the saturation of each sample to the log-indicated fractional water saturation at the formation level where the sample was obtained. If the former is about equal to or exceeds the latter, it is likely that the water in the formation is bound whereas if the latter exceeds the former, free water exists which will be produced if the formation is tapped at that location. Gathering and analyzing formation samples is both time consuming and expensive. In a related method, the bound water saturation determined from the gathered samples are used to determine a so-called "Productive Resistivity" corresponding to that of a low water cut hydrocarbon reservoir which is then compared directly with the formation resistivity log data. This method is described in the aforesaid Gauntt et al article.
The number of samples needed to determine irreducible water saturation of a formation is reduced in some methods by empirically relating the bound or irreducible water saturation measured from a small group of representative formation samples to a characteristic of the formation which can be measured with or determined from the output of conventional logging tools. These methods are based upon the widely held view that bound water saturations are related in some manner to such characteristics of the formation as its porosity, permeability and/or specific surface area (i.e. surface area per quantity of the formation material). Some of these relationships are summarized by A. Timur in an article entitled "An Investigation of Permeability, Porosity, & Residual Water Saturation Relationships for Sandstone Reservoirs," THE LOG ANALYSTS, pp. 8-17, July-August, 1968. One such method, for example, is to correlate irreducible water saturations, surface areas or both measured from rock samples to a formation shaliness factor determined from conventional Spontaneous Potential, gamma ray or density and velocity logs, and is described by Murphy and Owens in "A New Approach for Low-Resistivity Sand Log Analysis", JOUR. OF PETROLEUM TECH., pp. 1302-1306, November 1972.
Copending application Ser. No. 339,963 filed Jan. 18, 1982, assigned to the same assignee as this application, describes a method of deriving a log of specific surface area of the formation material from which potential low water cut pay may be located in sandstone formations. The specific surface area log is derived from permeability and porosity logs derived from the output of an acoustic or sonic logging tool or such a tool and one or more other tools. The method does not require gathering formation samples if the acoustic log has been previously calibrated to the permeability of the formation. Only one or a few samples need be gathered to accomplish calibration if it is required.
Most pertinent to this application, one method has been reported in the literature for determining irreducible water saturation from log-indicated formation porosity and fractional water saturation values. See LOG INTERPRETATION Volume I-Principles, Schlumberger Ltd., New York, N.Y., 1972 Ed., page 104. It is there reported that for a given rock type, plots of porosity and fractional water saturation determined from logs of a given formation "fall in a fairly coherent pattern approximating a hyperbolic curve" of the form S.sub.wirr =C/.phi., where S.sub.wirr is the irreducible water saturation, C is a constant and .phi. is the formation porosity. The function is determined by plotting pairs of log-indicated formation porosity and water saturation values on a common graph. Those porosity/water saturation points which are closest to the origin of the graph will conform roughly to the hyperbolic curve representing the irreducible water saturation of the formation.